1. Field of the Invention
The present invention relates in general to a device and a method for measuring and adjusting pressing load values on a press-forming machine, and more particularly to a device capable of measuring the pressing load values with high accuracy using strain sensors, and a method of adjusting local pressing load values to optimum values independently of each other so as to establish a predetermined local load distribution of the press.
2. Discussion of the Related Art
A press having a vertically reciprocating slide with an upper die is widely used for producing bodies of an automotive vehicle and other articles. For adjusting or monitoring load values on such a press, it is practiced to attach suitable strain sensors on the frame members which support a press slide, and measure the load values on the press on the basis of the amounts of elongation of the frame members. The strain sensors may be attached to suitable members of a drive mechanism for reciprocating the press slide. The measurement of the load values is based on the assumption that a pressing force F is proportional to the amount of strain .epsilon.. The pressing force F is obtained according to the following equation (1): EQU F=a.epsilon. (1)
where, a=predetermined proportion constant.
JP-A-57-30919 (laid-open publication of Japanese Patent Application) discloses an example of a device operated on the above assumption, wherein a permanent magnet and a Hall element are used as a strain sensor. The strain sensor may be any other suitable means for detecting the amount of elongation of a frame member of the press, which may be of dial gage type, electrical capacitance type, strain gage type, optical type, differential transformer type or rotary encoder type.
However, the pressing force F at a given position on a press slide adapted to perform a pressing action with an upper and a lower die is not necessarily sufficiently linearly proportional to the amount of strain of the frame members, due to various fluctuating factors such as positional errors associated with the press slide, die set and frame members, specific configurations of the frame members, and gear backlash of the drive mechanism to drive the press slide. Accordingly, the amount of strain .epsilon. as detected by the strain gages will not accurately represent the actual local pressing force value F. Solid and one-dot chain lines in the graph of FIG. 7 represent examples of an actual local pressing force value Fpi, and the level of an output signal Si of a strain sensor, respectively. It will be understood from the graph that although the pressing force value Fpi and the strain sensor output level Si may be partially coincident with each other by the use of a suitable proportion constant .alpha., these two values are not linearly proportional with each other over the entire range of the effective pressing stroke of the press slide.
When the press has two or more frame members, a strain sensor is attached to each of the frame members. In this case, the total pressing load is equal to a sum of the local pressing load values as represented by the outputs of the individual strain sensors. Usually, a proportion constant a is used for the total output of the strain sensors to obtain the total pressing load from the total sensor output. This proportion constant a is also used to obtain the local pressing load values from the outputs of the individual strain sensors. Therefore, the obtained local pressing load values are not necessarily sufficiently accurate. Accordingly, for example, it is not possible to precisely detect the distribution of local holding forces which act on the workpiece or blank at the respective local portions of the press slide or die set. Further, the detected local pressing load values do not permit accurate adjustment of the local pressing load values to the optimum values as found or established on a test press when the die set is prepared.
To establish the optimum local pressing load values suitable for the specific die set used, a press is usually provided with a plurality of height-adjusting mechanisms at respective local positions on the press, for adjusting a relative distance (indicated at h1 in FIG. 2, for example) between the press slide and the drive mechanism. This height-adjusting mechanisms permit efficient reproduction of the optimum local pressing load values on the press in a production line.
However, the relationship between the actual local load value and the relative distance as defined above at one local position on the press differs from those at the other local positions, due to plays and gear backlash of the drive mechanism for the press slide, parallelism variation of the press slide, and dimensional errors and rigidity variation of the individual components of the press. Consequently, while the height-adjusting mechanisms permit the adjustment of the total pressing load, the distribution of the local load values differs on the individual pressing machines, whereby the articles produced by the individual machines do not necessarily have an intended quality, even if the total load is adequately adjusted. In this respect, it is considered possible to adjust the press depending upon the specific die set used, so as to obtain the intended quality of the product. Where the press in question is used with the different die sets for producing different articles, the adjustment of the press for one die set will influence the quality of the articles to be produced by the other die sets. In view of this fact, it is generally practiced to adjust the die set per se depending upon the characteristics of the press.